Ecology Lecture 10 Life History Patterns 1. Topics covered (both Life History Lectures) Sexual selection What criteria do individuals use to choose

Ecology Lecture 10Ecology Lecture 10

Life History Patterns 1

Topics covered (both Life Topics covered (both Life History Lectures)History Lectures)

Sexual selection What criteria do individuals use to choose

mates?

Mating systems How many mates does an individual have What factors determine this?

Parental care/parental investment in offspring

Allocation of resources Quick reproduction vs. growth & long life (r

and K strategists, etc…)

Relevance to Population Relevance to Population EcologyEcology

Life-history characteristics determine Organization of individuals in space and time

Examples…

How the population perpetuates itself discretely vs. continuously; rapidly vs. slowly, etc…

Which characteristics will be selected for and persist within the population (due to sexual selection)

Conversely, selective pressures (abiotic factors, distribution of resources, etc..) will affect the life-history patterns observed

Sexual selectionSexual selection

Defined: Selection for characteristics/behaviors

that maximize chances of mating and producing the most, and highest quality, offspring.

A category within “natural selection” General pattern: Male-male

competition and female choice Common pattern with many

exceptions!

Example: Satin bowerbird Example: Satin bowerbird mating behaviormating behavior

Each male build an elaborate bower where he conducts his courtship display

Bowers located near each other

Each female visits several times, finally chooses a mate

Bowerbird males: # of Bowerbird males: # of matesmates

Some males much more successful than others

Bowerbird females: # of Bowerbird females: # of matesmates

Only ~1/3 of females have >1 mate

Why do males usually Why do males usually compete, while females compete, while females

choose?choose? Hypothesis 1, A.J. Bateman: “Eggs are

expensive, sperm is cheap!”

Amount of energy invested in a single gamete is much greater for females (eggs) than for males (sperm) Female bird may

invest up to 30% of body weight in eggs.

““Eggs are expensive, Eggs are expensive, sperm is cheap.”sperm is cheap.”

Difference in investment per gamete Example: Fairy wrens

Males have 8 billion sperm in testes at once Females lay six eggs maximum per clutch

Female limited by egg production, Male limited by number of mates

only (presumably unlimited sperm) Operational sex ratio skewed

toward males

Is sperm really cheap?Is sperm really cheap?

It often takes a large number of sperm to fertilize a single egg, due to

Hostile environment within female Acid Attacks by the immune system


Sperm competition among males Occurs when females have multiple

mates Possibly the predominant situation Fitness advantages for female (will explore in

next lecture)

Some males may actually run out of sperm… Garter snakes, zebra finch, blue crabs,

rams…

What if there is no sperm What if there is no sperm competition?competition?

In sea horses, eggs are deposited into pouches, and there is not sperm competition. Why not?

Male sea horses have relatively low sperm counts!


Drosophila bifurca: one sperm with long tail Sperm tail is 20x

length of his body His testes make up

11% of his body mass.

Why do males usually Why do males usually compete, while females compete, while females

choose?choose? Hypothesis 2 (R.

Trivers): Competition vs. choice is based on individual with the most total parental investment

Often the female (example: mammals)

But in some species, male makes a greater total investment

Gulf pipefishGulf pipefish

While male cares for a single brood, a female can produce two clutches of eggs male has greater total parental investment Operational sex ratio skewed toward females.

Males choose large, ( ornamented females over small, drab ones.

Female

Male

Types of sexual selectionTypes of sexual selection

Intrasexual selection: maintenance of traits that assist in competition within the gender successful mating

Intersexual selection: maintenance of traits that are attractive to the opposite gender

Intrasexual selection 1: Intrasexual selection 1: Adaptations to gain access to Adaptations to gain access to

femalesfemales

Dominance behavior & characteristics Example 1: sexual dimorphism in

elephant seals

Male-male competition and sexual Male-male competition and sexual dimorphism (seals)dimorphism (seals)

Male dung beetle, Phanaeus vindex (Rattlebox photography)

Intrasexual selection 1: Intrasexual selection 1: Adaptations to gain access to Adaptations to gain access to

femalesfemales Weaponry for fighting with other males.

Example: dung beetle!

IntraIntrasexual selection 1: sexual selection 1: Adaptations to gain access to Adaptations to gain access to

femalesfemales Sneaker

strategies Example 1: Plainfin

midshipmen Dominant male

features and behavior Nest building, singing,

guarding Sneaker male features

and behavior No nest, no singing,

just sex… Small fish, big balls!

Dung beetles: two Dung beetles: two morphsmorphs

Behavioral and morphologial differences similar to midshipmen Large, dominant males with

horns defend burrows Small, hornless males with

“big balls” sneak Midshipmen

Genetically-based differences Dung beetles

Nutritionally-based differences

Intrasexual selection 2: Intrasexual selection 2: Adaptations favoring the use of one’s Adaptations favoring the use of one’s

spermsperm

Displacing or inactivating rival sperm Damselfly “scooper” penis


spermsperm

Displacing or inactivating rival sperm Example: Chemical sperm inactivation in

fruit flies

Photo: San Francisco Exploratorium


spermsperm

Mechanisms to avoid sperm displacement Mate guarding

(Example: many crab species)


spermsperm

Mechanisms to avoid sperm displacement Prolonged mating and cannibalism (example: redback

spider)

Female less likely to mate with another if she eats him

He has low likelihood of finding a new mate (high predation) Andrade, 1996


spermsperm

Mechanisms to avoid sperm displacement Anti-aphrodisiac (Example: Heliconius erato)

InterIntersexual selectionsexual selectionFocus on female choiceFocus on female choice

Material benefits: Nutrition Ex: hangflies) Length of mating time depends on quality and size

of “courtship gift”


Material benefits: Anti-predator substances Defensive compounds in arctiid moths

Photo: butterfly-conservation.org


Ability of males to provide sufficient sperm Female fruit flies (some species) choose virgin males

Photo: San Francisco Exploratorium


Parental ability Cannot assess

directly May be correlated

with other features of the male

Example 1: Redwing blackbird “Epaulettes” correlated

with nest defense Courtship effort

correlated with feeding effort

Photo: Vancouverislandbirds.com


Parental ability (Ex: sedge warbler) Size of song repertoire correlates with chick

weight at fledging

Sedge warbler (cont.)Sedge warbler (cont.)

Female sedge warblers choose on the basis of repertoire size. Thus they choose

the most fit males


Health/Genetic quality Example: song repertoire in great reed warbler Females chose males with larger repertoires This was correlated with greater offspring

survival (unrelated to parental care)


Health/Genetic quality Example: Bright coloration of sticklebacks

negatively correlated with low parasite loads


Health/Genetic quality (bright color negatively correlated with parasite load) Advantages to females choosing these

males Avoid getting parasites while mating Avoid transferring parasites to young Are choosing healthier males; their health

status may be related to “genetic quality”

Satin Bowerbirds: multiple signals Satin Bowerbirds: multiple signals of health and fitness (and good of health and fitness (and good

genes?)genes?)

Origin/maintenance of mate Origin/maintenance of mate choice for “exaggerated” choice for “exaggerated”

characteristicscharacteristics In some species, why do males

develop what appear to be extreme traits that actually can hamper their survival?

Example: Peacock’s tail


characteristicscharacteristics Hypothesis 1: Runaway selection

(R.A. Fisher) Directional that takes on a life of its own

Starts as an “honest signal” more extreme. Mechanism: Females choose males with

large tails, multiple eyespots. the next generation has a higher proportion of these males. Will work even if his traits are not honest signals of

quality. Why? Evidence of arbitrary choices by females (bird band

example)


characteristicscharacteristics Hypothesis 2: Handicap or “good

genes” hypothesis (R.A. Fisher) Exaggerated trait might decrease chance

of survival, only males with superior genes can survive despite the handicap Example: peacock tail as a handicap.

In this case, a female choosing a male with these traits would be improving her fitness. (His signal is an “honest signal” of fitness.)

Distinguishing between Runaway Distinguishing between Runaway Selection and “Good Genes” Selection and “Good Genes”

(Petrie)(Petrie)

Methods Males of different ornamentation/tail length

and randomly bred them with females Why random?

Young raised under identical conditions and then released

Results Offspring of the “attractive” males weighed

more at day 84. Offspring of the attractive males were more

likely to be alive after two years Which hypothesis is supported by

this data?

Petrie’s peacock dataPetrie’s peacock data


characteristicscharacteristics How extreme can a

characteristic become? Under what conditions will directional selection stop? (Think about costs vs. benefits…)

Documents

Ecology Lecture 10 Life History Patterns 1. Topics covered (both Life History Lectures) Sexual selection What criteria do individuals use to choose